Using a device consisting of an acoustical driver, electroacoustical sensor and negative feedback of the sensor output to the driver, acoustic noise can be reduced in the vicinity of the acoustical driver. Such a device is shown in "Electronic Control of Noise, Vibration and Reverberation," by H. F. Olson, J. Acoust. Soc. Amer., Vol. 28, No. 5, pp. 966-972, September 1956. This reference describes a device consisting of a speaker, which converts an electrical signal into an acoustical output, a microphone, which converts sound pressure into an electrical signal, and an amplifier which feeds back the electrical signal of the microphone to the speaker in a negative fashion. The effect of the negative feedback of the microphone signal is to reduce the sound pressure at the microphone. This reference describes several potential applications, such as reducing sound in small enclosed cavities, headphones, and helmets, and in providing localized noise reduction, such as near a machine operator's head or near the head of a passenger of an airplane or automobile.
An approach to noise reduction in an airplane also is described in "Active Sound Attenuation Across a Double Wall Structure," in Journal of Aircraft, Vol 31, No. 1, January-February 1994, pp. 223-227, by F. W. Grosveld and K. P. Shepherd. This system also is described in U.S. Pat. No. 5,024,288. This reference describes a device in which the microphone is not placed in the vicinity of the acoustical driver. Rather, acoustic drivers are placed in what is known as the trim panel cavity of the aircraft. Error microphones are placed in the cabin. A feedforward approach is used to drive the acoustical drivers. This feedforward approach requires the error microphones to be in the cabin and also requires a coherent reference signal which may be impractical or unfeasible. This feedforward approach also tends to amplify noise at locations away from the error microphones.